In the processing of fabrics, it is frequently necessary or desirable to impregnate the fabric with chemicals, such as fabric softeners, resins, fabric performance chemicals, stain resists, resins, fabric performance chemicals, etc. This is conveniently done while the fabric is in web form, when it can be run continuously through a solution bath. As the fabric exits the solution, it typically is passed through a pair of extraction rollers which squeeze out the excess processing liquid. The impregnated fabric is then dried, or dried and cured, or dried and cured, and subjected to further processing.
Inasmuch as wet fabric more easily absorbs processing solution, it is typical for the fabric to be wet-out prior to being immersed in the processing solution. The wet-out procedure desirably is performed closely upstream of the chemical processing, in order that the fabric entering the processing solution will have a substantially uniform and controlled content of water.
When the wet-out fabric enters the bath of chemical solution, it absorbs excess quantities of the solution. As the fabric exits the solution, and passes through the extraction rollers, excess liquid is pressed out of the fabric and falls back into the solution pan, and the fabric emerging from the exit side of the extraction rollers carries away a predetermined, uniform quantity of the solution. Inasmuch as the fabric entering the solution bath already carries a significant quantity of water, the process of impregnating the fabric in the solution bath and then extracting excess solution upon exit tends to result in a progressive dilution of the solution bath over time, because more water is being brought in to the solution than is being carried away by the processed fabric.
With conventional and accepted practices, it is customary to process on a batch basis, in which a quantity of the processing solution is prepared in a pre-mix tank and is supplied from the pre-mix tank to a processing pan, through which the fabric is guided. During processing, the pre-mixed solution is periodically supplied from the pre-mix tank to the processing pan, according to a level control facility associated with the pan. Because of the continuous dilution of the chemical solution in the processing pan during operations, it is customary to provide a solution pre-mix that contains a substantially higher concentration of the process chemical than is required for the fabric. For example, where the desired solution contains 2.5%-3% solids, the conventional pre-mix solution will have a typical concentration of about 7% solids. The initial run of fabric passing through the solution will thus pick up the processing chemical at the 7% level, which of course imparts to the fabric a much greater quantity of the chemical than is needed (or even desired). Over a period of time, for example, 20-30 minutes, the solids concentration in the solution pan becomes progressively less, as the periodic replenishment of the solution pan with 7% solids mixes with a progressively more dilute solution in the pan resulting from water introduced from the incoming fabric. After an initial 20-30 minute period, a substantial equilibrium is reached, at which the periodic replenishments from the 7% solids pre-mix solution are substantially balanced by the incoming dilution, as wet fabric continues to enter the solution pan, adding water, and chemical and water are carried away by the processed fabric.
In conventional processing, it is also generally considered necessary for the above described equilibrium level of the processing solution to be set at a level somewhat higher than the desired specification for the processed fabric, because of the many variables that enter into the eventual equilibrium condition. Among other things, the processing bath is replenished periodically with a 7% solids solution. Also, the time to reach an equilibrium point can vary as a function of the width and weight of the fabric being processed, as well as a function of the throughput speed.
The losses to the fabric processor resulting from overapplication of processing chemicals, as described above for conventional processing, can be truly staggering. In a processing operation having a production rate of 200,000 pounds per week treated with a soil release chemical, for example, direct losses from overapplication of the soil release chemical, based upon chemical costs alone, can exceed $400,000 per year. Moreover, the losses from such overapplication of chemicals are not limited to the chemical costs, but involve downstream processing as well. For example, during the drying of fabric after chemical processing, some of the chemical becomes deposited internally in the dryer, necessitating occasional maintenance cleaning. Where excess quantities of the chemical are being carried by the fabric, maintenance cleaning must be done more frequently, with resulting expense and downtime. Additionally, subsequent compressive shrinking operation are less effective and less satisfactory when the fabric is carrying excessive amounts of processing chemicals.
Under-application of chemicals to a fabric can also result in costly losses. For example, under-application of chemical over a portion of the fabric, may result in the fabric being not up to quality control specifications and rejected on that basis. With conventional processing, the solution becomes progressively more diluted over time and in many cases is diluted to a level below the percent solids which is standard for the process. In those cases, it frequently is necessary to stop the process, drain and discard the dilute solution, and refill the pan with new solution. This, of course, requires a new equilibrium period to take place, as the solids concentration of the new solution gradually dilutes toward the desired standard percentage of solids. Some processors may discard as much as $2000.00 per week of solution which has become diluted below standard as regards percent solids. Stoppage of the continuous processing line during drain and refill procedure further exacerbates the losses experienced by the processor. With the system of the present invention, losses of this type are reliably avoided, because the processing solution can be maintained on specification with a high degree of accuracy and reliability.
An additional advantage of the invention is derived from the fact that certain procedures, which are now performed on a wet-on-dry basis, because of difficulty in performing them with conventional wet-on-wet procedures, can now be performed to significant advantages as wet-on-wet procedures. Among other things, this saves the cost and time of performing an intermediate drying step on the fabric, prior to performing wet-on-dry chemical processing. Additionally, both fabric strength and shrinkage are improved with wet-on-wet application, as compared to wet-on-dry processing.